A point source of light `S` is placed at the bottom of a vessel containing a liquid of refractive index `5//3`. A person is viewing the source from above the surface. There is an opaque disc of radius `1 cm` floating on the surface. The centre of disc lies vertically above the source `O`. The liquid from the vessel is gradually drained out through a tap. What is the maximum height of the liquid for which the source cannot be seen at all.

Explanation

To find the maximum height of the liquid for which the source cannot be seen at all, we need to analyze the situation using the concept of critical angle and Snell's law.

### Step-by-Step Solution:

1. **Understanding the Setup**:
- We have a point source of light `S` at the bottom of a vessel filled with a liquid of refractive index \( \mu = \frac{5}{3} \).
- An opaque disc of radius \( R = 1 \, \text{cm} \) is floating on the surface of the liquid, directly above the source.

...